The present invention relates to a two-color thermosensitive recording adhesive label and more particularly to a two-color thermosensitive recording adhesive label comprising a support material, two thermosensitive coloring layers successively formed on one side of the support material, each coloring layer being capable of yielding a different color by application of a different quantity of thermal energy thereto, an adhesive layer formed on the other side of the support material, opposite to the thermosensitive coloring layers, and a disposable backing sheet which is attached to the adhesive layer and can be peeled off the adhesive layer when the thermosensitive recording adhesive label is used.
A conventional thermosensitive recording material comprises a support material such as a sheet of paper and a thermosensitive coloring layer formed on the support material, on which thermosensitive coloring layer colored images can be formed by application of heat thereto. For image formation by application of heat, a thermal printer provided with a thermal head is in general use. In such a conventional thermosensitive recording material, there are usually employed in the thermosensitive coloring layer a colorless or light-colored leuco dye containing a lactone ring, a lactam ring or a spiropyran ring, and a color developer which induces color formation in the leuco dye upon application of heat by the reaction with the leuco dye, since it is capable of yielding clear images with minimized fogging.
Because of the capability of forming colored images by simple application of heat, such thermosensitive recording materials are widely used, not only for copying books and documents, but also for recording output information from computers, facsimile apparatus, telex and other information transmission and measuring instruments. Conventionally, the color of the images developed in the thermosensitive recording materials is mostly a mono color such as blue and black. Depending upon the recording mode, it will be more convenient if it is allowed to record particular data in a different color from the remainder on a thermosensitive recording material in order to display the particular data more distinctly from the remainder.
Recently, many trials have been made to attain recording with multiple colors by applying heat at different temperatures or by applying different quantities of thermal energy. Accordingly, a variety of multi-color thermosensitive recording sheets have been proposed.
A conventional multi-color thermosensitive recording sheet comprises a support material and two thermosensitive color-forming layers overlaying the support material, which color-forming layers are colored in different colors upon application of different thermal energies thereto respectively. One layer is referred to as, for example, a high-temperature color-forming layer and the other is referred to as, for example, a low-temperature color-forming layer. The low-temperature color-forming layer forms a color at a low temperature, while the high-temperature color-forming layer does not form a color at all at the low temperature, but forms a color at a high temperature which is higher than the low temperature, and the two colors are different from each other.
Such conventional multi-color thermosensitive recording sheets can be roughly classified into the following two types.
In one type, when a high-temperature color-forming layer is colored by application of heat at a high temperature, the color developed in the high-temperature color-forming layer is mixed with the color already developed in a low-temperature color-forming layer, so that a different color from the color in the low-temperature layer is produced in the high-temperature color-forming layer.
In the other type, when the high-temperature color-forming layer is colored, the color in the low-temperature color-forming layer is decolorized by a decolorizing agent, so that only the high-temperature color-forming layer is colored without the mixing of the color of the low-temperature color-forming layer and the color of the high-temperature color-forming layer.
Specific examples of the former type are disclosed, for instance, in Japanese Patent Publications Nos. 49-69, 49-4342 and 49-27708, Japanese Laid-Open Patent Applications No. 48-86543 and No. 49-65239.
Specific examples of the latter type are disclosed, for instance, in Japanese Patent Publications Nos. 50-17865, 50-17866, 51-29024, 51-87542, 54-36864 and 55-36519.
The former type has the shortcoming that the practically developable color systems are limited to such combinations that the color developed at high temperature can overcome the color developed at low temperature, such as red (low temperature) - black (high temperature), and blue (low temperature) - black (high temperature).
In the latter type, there are no particular limitations to the combination of colors. However, a decolorizing agent capable of completely decolorizing the color developed in the low-temperature coloring color-forming layer (when developing a color in the high temperature color-forming layer) has not been discovered, so that the separation of the color in the high temperature color-forming layer from the color in the low temperature color-forming layer is incomplete. The result is that the color developed in the low temperature color-forming layer spreads and overlaps the color developed in the high temperature color-forming layer. In order to avoid such problem, the following compounds are disclosed in Japanese Patent Publication No. 51-19992: acetamide, stearamide, phthalonitrile, m-nitroaniline and .beta.-naphthylamine. Of these compounds, stearamide and phthalonitrile are generally employed as sensitizer agent and their decolorizing effect is slight. By contrast, m-nitroaniline and .beta.-naphthylamine have relatively high decolorizing effect. However, they are apt to be decomposed to become yellowish in color and are highly soluble in water. Therefore, the images developed in the low temperature color-forming layer are easily decolored by m-nitroaniline and .beta.-naphthylamine. Furthermore, they are highly toxic and absorbed through skin, causing eye and skin irritations. Therefore the contact with skin, eyes and clothing must be avoided. In this sense, they are not suitable for use in practice.
In Japanese Patent Publication No. 54-36864, quaternary ammonium salts (amine derivatives) are proposed as decolorizing agent. However, the quaternary ammonium salts are highly water-soluble and are not suitable for use in practice from the viewpoint of the capability of stably preserving the developed images. In addition to the above, amine derivatives such as hexadecylamine, tribenzylamine, tricyclohexylamine, dioctadecylamine, N,N-dibenzylpiperazine and cychohexylbenzylamine are proposed. However, primary amines are unstable in the air and secondary and tertially amines having substituents such as alkyl group, aryl group and aralkyl group at the nitrogen atom are poor in preservability. Therefore, these amine derivatives are not suitable for use in practice.
In Japanese Patent Publication No. 51-29024, guanidine derivatives are proposed. However, guanidine is highly water-soluble. Therefore it is necessary to improve its preservability by making it insoluble or slightly soluble in water by bonding hydrophobic groups such as phenyl group and cyclohexyl group to guanidine or by making it a dimer. These derivatives have a decolorizing effect to some extent, but have the shortcomings that they are prone to be thermally decomposed, forming gases, and have poor preservability.
Therefore, it is preferable that a decolorizing agent for use in practice be basic, insoluble or slightly soluble in water, have a melting point of 80.degree. C. or more, stable in the air and to heat, and the toxicity of the agent be very low.
Conventional thermosensitive recording adhesive labels are widely used, mainly as bar-code labels in the POS system. Recently they are also used in delivery systems in the fields of food and related industries and mail business, and in the automatic transportation systems for warehouse and the like. Thermosensitive recording bar code labels have the advantages over the preprinted bar code labels that the desired codes can be printed on the spot and they can be simultaneously controlled by computer systems. Recently it has been proposed to make a particular portion (for instance, a corrected portion) in a label attract attention by printing such portion with a color different from black. Therefore, there is a demand for a thermosensitive recording adhesive label capable of meeting the above proposal.
Further, conventional two-color thermosensitive recording adhesive labels have the shortcomings that the image density obtained in the low temperature color-forming layer is low and finger print coloring takes place in the high temperature coloring layer. When it is tried to increase the image density of the images developed in the low temperature color-forming layer, the decolorizing of the images becomes insufficient when image formation in the high temperature color-forming layer is carried out, so that color mixing inevitably takes place.